There is a need for containers of thermoplastic material with superior configurational stability also at high temperatures, for example for the storage of foods. In particular in applications in which pasteurization, hot-filling or sterilization occur, it is necessary that the containers, without configurational change, withstand elevated temperatures. It is also obvious that, in applications in which the containers are filled with, for example, beverages containing, for instance, carbonic acid or nitrogen gas, high pressures occur in the sealed container both in connection with the filling and during subsequent storage. In particular, it should be observed that containers filled with carbonated beverages reach very high pressures on heating. As a non-restrictive example of thermoplastic material for containers for the above-indicated purposes, polyethylene terephthalate is disclosed, hereinafter generally abbreviated to PET. The thermoplastic material PET like most of the thermoplastic materials, is int. al. characterized in that it obtains good mechanical strength properties by mono and/or biaxial orientation, the material is thermocrystallizable and that its barrier properties are as a rule sufficiently good for many applications within the food sector.
As has already briefly been disclosed above, the mono and/or biaxial orientation of the thermoplastic material entails at demands placed on configurational strength and configurational stability are achieved in those parts of the container in which oriented material is included. In the production of the container, only the material in the walls of the container body is oriented, the material being, as a rule, given biaxial orientation. However, in certain applications the material in the mouth portion of the container is also oriented. On the other hand, in most applications, the material in the bottom portion of the container is not oriented, at least not in central regions of the bottom portion. It is previously known to thermally crystallize such non-oriented material portions in order to increase configurational stability both at low temperatures and at elevated temperatures. Since the material, before the thermocrystallization, is substantially amorphous, because of the fact that the material has not been oriented or oriented to but a slight degree, the material will as a rule be of relatively great thickness, which, in many applications, essentially corresponds to the original material thickness of the blank which is in the process of being reformed into the container. The term "substantially amorphous" signifies that the material has a crystallinity of at most approx. 10%.
Because of the poor thermal conductivity of the thermoplastic material, in combination with the relatively large wall thickness, heating for a lengthy time is required in order that the material will achieve the desired crystallinity. In applications in which the material is crystallized thermally by abutment against hot tool sur faces, abutment times of the order of magnitude of 15-30 s are as a rule required. It is obvious that the long abutment times reduce production capacity of the equipment employed, which leads to un desirably high capital costs for the production of the containers.
A further disadvantage in prior art technique is that, in conjunction with thermal crystallization of substantially amorphous material in the bottom portion of a container, the material in this will have a crystallinity which varies from region to region, and in addition, in certain areas the bottom portion will also have an undesirable deformation.
The varying crystallinity is because gas mixtures, as a rule air, are enclosed between the hot tool surfaces and the thermoplastic material. The enclosed gas mixtures form pockets between the tool surfaces and the plastic material. Each pocket constitutes an insulating layer which reduces the thermal exchange between the tool surfaces and the thermoplastic material, whereby, on heating of the material, that material in the area of each respective pocket is supplied with a smaller quantity of energy than in those regions where the material abuts against the tool surfaces. Thereby, the plastic material in the region of each respective pocket will reach an insufficiently high temperature, or the requisite high temperature for such a short time that the total thermal crystallinity in the material will be far too slight. The unevenly distributed crystallinity entails that the crystalline material will have poor configurational stability in the the bottom portion.
The undesirable deformation is a consequence of the fact that the gas mixture enclosed in each respective pocket is heated, excess pressure being formed in the pocket. Since the thermoplastic material, despite the insulating effect of the pocket, reaches a sufficiently high temperature to soften, the thermoplastic material in the region of the pocket is displaced, by the excess pressure in the pocket, in a direction towards the interior of the container and/or the material is attenuated, the undesirable deformation occurring. It is obvious that the bottom portion in the thinner material regions will have reduced mechanical strength.
In those areas of the bottom portion where there is a gravure, for example a company trade mark, a patent number, etc, the above-indicated problems with insufficient and/or varying crystallinity and undesirable deformation, respectively, may readily occur.
The present invention discloses a method and an apparatus in which the above-indicated desiderata have been satisfied and the above-outlined drawbacks have been obviated.